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How to troubleshoot security error codes on secure websites

On websites which are supposed to be secure (the URL begins with "https://"), Firefox must verify that the certificate presented by the website is valid. If the certificate cannot be validated, Firefox will stop the connection to the website and show a "Your connection is not secure""Warning: Potential Security Risk Ahead" error page instead.

This article explains why you might see SEC_ERROR_UNKNOWN_ISSUER, MOZILLA_PKIX_ERROR_MITM_DETECTED or ERROR_SELF_SIGNED_CERT error codes on an error page and how to troubleshoot it.

What does this error code mean?

During a secure connection, a website must provide a certificate issued by a trusted certificate authority to ensure that the user is connected to the intended target and the connection is encrypted. If you click the Advanced button on a "Your connection is not secure""Warning: Potential Security Risk Ahead" error page and you see the error code SEC_ERROR_UNKNOWN_ISSUER or MOZILLA_PKIX_ERROR_MITM_DETECTED, it means that the provided certificate was issued by a certificate authority that is not known by Firefox and therefore cannot be trusted by default.

The error occurs on multiple secure sites

In case you get this problem on multiple unrelated HTTPS-sites, it indicates that something on your system or network is intercepting your connection and injecting certificates in a way that is not trusted by Firefox. The most common causes are security software scanning encrypted connections or malware listening in, replacing legitimate website certificates with their own. In particular, the error code MOZILLA_PKIX_ERROR_MITM_DETECTED indicates that Firefox is able to detect that the connection is intercepted.

Antivirus products

Third-party antivirus software can interfere with Firefox's secure connections. We recommend uninstalling your third-party software and using the security software offered for Windows by Microsoft:

Bullguard

In Bullguard security products you can disable the interception of secure connections on particular major websites like Google, Yahoo and Facebook:

Open the dashboard of your Bullguard application.

Click on Settings and enable the Advanced view on the top right of the panel.

Go to Antivirus > Safe browsing.

Uncheck the Show safe results option for those websites which are showing an error message.

ESET

In ESET security products you can try to disable and re-enable SSL/TLS protocol filtering or generally disable the interception of secure connections as described in ESET’s support article.

Kaspersky

Affected users of Kaspersky should upgrade to the most recent version of their security product, as Kaspersky 2019 and above contain mitigations for this problem. The Kaspersky Downloads page includes "update" links that will install the latest version free of charge for users with a current subscription.

Otherwise, you can also disable the interception of secure connections:

Open the dashboard of your Kaspersky application.

Click on Settings on the bottom-left.

Click Additional and then Network.

In the Encrypted connections scanning section check the Do not scan encrypted connections option and confirm this change.

Finally, reboot your system for the changes to take effect.

Family Safety settings in Windows accounts

In Microsoft Windows accounts protected by Family Safety settings, secure connections on popular websites like Google, Facebook and YouTube might be intercepted and their certificates replaced by a certificate issued by Microsoft in order to filter and record search activity.

Read this Microsoft FAQ page on how to turn off these family features for accounts. In case you want to manually install the missing certificates for affected accounts, you can refer to this Microsoft support article.

Monitoring/filtering in corporate networks

Some traffic monitoring/filtering products used in corporate environments might intercept encrypted connections by replacing a website's certificate with their own, at the same time possibly triggering errors on secure HTTPS-sites.

If you suspect this might be the case, please contact your IT department to ensure the correct configuration of Firefox to enable it working properly in such an environment, as the necessary certificate might have to be placed in the Firefox trust store first. More information for IT departments on how to go about this can be found in the Mozilla Wiki page CA:AddRootToFirefox.

Malware

The error occurs on one particular site only

In case you get this problem on one particular site only, this type of error generally indicates that the web server is not configured properly. However, if you see this error on a legitimate major website like Google or Facebook or sites where financial transactions take place, you should continue with the steps outlined above.

Certificate issued by a authority belonging to Symantec

After a number of irregularities with certificates issued by Symantec root authorities came to light, browser vendors including Mozilla are gradually removing trust from these certificates in their products. In a first step, Firefox 60 will no longer trust certificates chaining up to Symantec root authorities (including all Symantec brands GeoTrust, RapidSSL, Thawte, and VeriSign) which were issued before 2016-06-01. In Firefox 63 this removal of trust will be extended to all Symantec certificates regardless of their issuing date.

MOZILLA_PKIX_ERROR_ADDITIONAL_POLICY_CONSTRAINT_FAILED will be the primary error but with some servers, you may see the error code SEC_ERROR_UNKNOWN_ISSUER instead. In any case, if you come across such a site you should contact the owner of the website to inform them of that problem. We strongly encourage operators of affected sites to take immediate action to replace these certificates.

Missing intermediate certificate

On a site with a missing intermediate certificate you will see the following error description after you click on Advanced on the error page:

The certificate is not trusted because the issuer certificate is unknown.The server might not be sending the appropriate intermediate certificates.An additional root certificate may need to be imported.

The website's certificate might not have been issued by a trusted certificate authority itself and no complete certificate chain to a trusted authority was provided either (a so-called "intermediate certificate" is missing).
You can test if a site is properly configured by entering a website's address into a third-party tool like SSL Labs' test page. If it is returning the result "Chain issues: Incomplete", a proper intermediate certificate is missing.
You should contact the owner of the website you're having troubles accessing to inform them of that problem.

Self-signed certificate

On a site with a self-signed certificate you will see the error code ERROR_SELF_SIGNED_CERT and the following error description, after you click on Advanced on the error page:

The certificate is not trusted because it is self-signed.

A self-signed certificate that wasn't issued by a recognized certificate authority is not trusted by default. Self-signed certificates can make your data safe from eavesdroppers, but say nothing about who the recipient of the data is. This is common for intranet websites that aren't available publicly and you may bypass the warning for such sites.

Bypassing the warning

Warning: You should never add a certificate exception for a legitimate major website or sites where financial transactions take place – in this case an invalid certificate can be an indication that your connection is compromised by a third party.

If the website allows it, you can bypass the warning in order to visit the site, even thought its certificate is not being trusted by default:

On the warning page, click Advanced.

Click Add Exception…. The Add Security Exception dialog will appear.

Read the text describing the problems with the website. You can click View… in order to closer inspect the untrusted certificate as well.

Click Confirm Security Exception if you are sure you want to trust the site.